I do what you're suggesting quite frequently. I put a few drops of WD40 down into the bearing, and then I use a high pressure air hose to spin the fan real fast. I then take a paper towel and wipe out the gunk. I might repeat the process one more time. Then I put a drop or two of 3 in 1 oil in there and give it another good spin.
After having done this dozens of times with fans of various size and make, I would say that it restores fan life for a considerable period. Sometimes the fan will start chattering again after a year or so and I'll repeat the process. In only a few situations have I ever had to buy a new fan. It's not that I'm cheap, it's that finding a replacement fan can be an irritating process..especially if it's some goofy chipset fan with a unique mount. This process only takes ten minutes, and so far I've found it less time consuming than trying to find a replacement.
One exception I make is for power supply fans. I don't open those things up anymore. I typically just buy a new supply.